Lately research on a photoacoustic diagnostic apparatus that acquires biological function information utilizing the photoacoustic effect is progressing as an apparatus that images inside the organism non-invasively in the medical field. The photoacoustic effect is a phenomena where an acoustic wave is generated when pulsed light is irradiated onto an object, due to the absorption of the light propagated and diffused inside the object. The photoacoustic diagnostic apparatus detects a time-based change of the generated acoustic wave at a plurality of locations, mathematically analyzes, in other words, reconstructs the acquired signals, and three-dimensionally visualizes information related to the optical characteristic values inside the object. Thereby an internal tissue, which is a generation source of the generated acoustic wave, is imaged. One reconstruction method is the back projection method. Back projection is a calculation method where each reception signal is propagated in reverse and superimposed to specify a signal source, considering the propagation velocity of the sound inside the object. If near infrared light is used for the pulsed light, blood vessel images can be easily generated since the near infrared light can easily transmit through water, which constitutes most of an organism, and is easily absorbed by hemoglobin in the blood. Further, oxygen saturation in the blood, which is functional information, can be measured by comparing blood vessel images generated using pulsed light of different wavelengths. Since the oxygen saturation of the blood around a malignant tumor is assumed to be lower than that of the blood around a benign tumor, it is expected that a malignant/benign tumor can be distinguished by knowing the oxygen saturation.
As Expression (1) shows, the intensity p of a photoacoustic wave depends on the intensity ϕ of light that reaches the light absorber.p=Γ·ϕ·μa  (1)
Here Γ is a Grüneisen constant and μa is a light absorption coefficient. As Expression (1) shows, even if the light absorption coefficient of the light absorber is a large value, the intensity of the acoustic wave to be generated is low if low quantity light reaches the light absorber. Whereas even if the light absorption coefficient is not so large, the intensity of the acoustic wave to be generated is high if a high quantity of light reaches the light absorber.
A configuration applying this principle has been proposed. Patent Literature 1 is a configuration of disposing a reflection member on a surface of a photoacoustic detector. Non-patent Literature 1 is a configuration of irradiating light onto an object from a plurality of directions.